1. Field of the Invention
The present invention generally relates to methods and apparatuses for measuring vision or visual acuity and methods for analyzing the same. More specifically, the present invention relates to a method for the measurement of so-called “operating vision” or “operating visual acuity” and to a method for analyzing vision.
2. Description of the Related Art
Conventional visual-acuity measurement instruments are designed to measure vision at the time of measurement under stress-free conditions that may be quite different from our day-to-day stressful conditions. Vision, however, declines through continuous use of the eyes. For some people, vision degrades rapidly after staring at a book, a video screen, or the like for a certain period of time without blinking. For example, staring without blinking while driving on an expressway may involve the risk of deteriorating vision. In some people, vision may drop under such conditions from about 1.0 to about 0.3. It is known that such a tendency is strong among people suffering from so-called “dry eye”, in which the ocular surfaces tend to become dry. Thus, the vision measured with such a conventional visual-acuity measurement instrument, and the actual visual acuity when the eyes are experiencing stress (we refer to this type of visual acuity as “operating vision”) may be different from each other, and the difference varies considerably depending on the individual and also on the environment and the degree of stress. In other words, with a temporary or momentary vision alone, it is difficult to accurately measure a subject's operating vision which is the effective vision the subject can utilize. In light of this tendency for vision to decline due to staring and also in view of influences of diseases, such as dry eye, on vision a person can practically use, it is desirable to measure deteriorating vision over time.
An apparatus for measuring operating vision is disclosed in JP 2001-309887 A. Such a known apparatus has been put into practical use. However, some points to be improved have also become apparent.
For example, with a conventional apparatus, if the subject's response to a newly displayed optotype mark is correct, a smaller optotype mark is presented, and if the response is incorrect, a larger optotype mark is presented (see paragraph No. 0017 in JP 2001-309887 A mentioned above). The method disclosed in this document is based on the premise that blinking is involved and the operating vision is recovered by blinking, as shown in FIG. 2 of JP 2001-309887 A. In actual tests, however, the eye is forcibly kept open, for example, for about 30 to 60 seconds in order to accurately measure the operating vision. Further, based on the premise that the operating vision decreases during the eye examination, a larger optotype mark is presented when an erroneous response is received, and optotype marks corresponding to the same vision are continuously presented when correct responses (i.e., correct answers) are continuously received.
Based on the premise that the operating vision decreases over time, a smaller optotype mark is not presented even when a correct response is received. This is because the operating vision measurement instruments were initially developed based on the assumptions that tear layers on the ocular surfaces deteriorate over time and measured visual acuity decreases over time. Thus, assuming that tear layers deteriorate unless blinking is allowed and the visual acuity tend to decrease over time, early operating vision measurement instruments were designed to display a larger optotype mark, which is to be read by the subject, for a visual acuity value which is for one level lower than the current vision level when the answer from the subject is incorrect or when no answer is given, and display an optotype mark corresponding to the same visual acuity value when the answer is correct.
Such an approach is reasonable if the eye is forcibly kept open. However, it has also become clear that subjects feel very uncomfortable to have their eyes forcibly kept open, which may involve the use of eye-drop anesthetic. In addition, it has become clear, with impatient subjects or subjects unfamiliar with the use of an input device, in particular, that the measurement results tend to indicate operating vision that is too low compared to the actual vision. More specifically, it has been known that, in many cases of actual measurement, the subjects inadvertently move a joystick even when they know the correct answers or the joystick does not operate properly as intended, thus leading to some errors. As a result, according to conventional measurements based on the premise that visual acuities decrease over time, measured operating vision tends to be lower than what they really are.